FIG. 1 illustrates an exemplary wireless communication system. Referring to FIG. 1, a wireless communication system 100 includes a plurality of Base Stations (BSs) 110a, 110b and 110c and a plurality of Mobile Stations (MSs) 120a to 120i. The wireless communication system 100 may include a homogeneous network or a heterogeneous network. A heterogeneous network refers to a network in which different network entities co-exist, such as macro cells, femto cells, pico cells, relays, etc. A BS is typically a fixed station that communicates with MSs and each BS 110a, 110b or 110c provides services to a specific geographical area 102a, 102b or 102c. To increase system performance, the specific area may be divided into a plurality of smaller areas 104a, 104b and 104c, which may be called cells, sectors, or segments. In an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system, a cell Identifier (ID) (IDcell) is assigned from the perspective of the entire system, whereas a sector ID or segment ID is assigned from the perspective of the coverage area of each BS, ranging from 0 to 2. The MSs 120a to 120i are fixed or mobile terminals that are usually distributed over the wireless communication system 100. Each MS may communicate with one or more BSs at a given time instant on a downlink and uplink. The communication between an MS and a BS may be carried out in Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Single Carrier-FDMA (SC-FDMA), Multi Carrier-FDMA (MC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or a combination of two or more of them. Herein, the term “uplink” refers to a communication link directed from an MS to a BS, and the term “downlink” refers to a communication link directed from a BS to an MS.
FIG. 2 illustrates the structure of a downlink subframe in an IEEE 802.16e system. The downlink frame structure was designed for operation in Time Division Duplex (TDD) mode.
Referring to FIG. 2, the downlink subframe includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols. The downlink subframe is divided in structure into a preamble, a Frame Control Header (FCH), a Downlink-MAP (DL-MAP), an Uplink-MAP (UL-MAP), and DL bursts. Each downlink subframe starts with a preamble that occupies the first OFDM symbol of the downlink subframe. The preamble is used for time synchronization acquisition, frequency synchronization acquisition, cell search, and channel estimation.
FIG. 3 illustrates subcarriers to which a preamble is mapped in 1024-Fast Fourier Transform (FFT) mode (a 10-MHz bandwidth) in the IEEE 802.16e system.
Referring to FIG. 3, some areas at both sides of a given bandwidth are used as guard bands. Therefore, the preamble is mapped to the rest except the guard bands. The remaining frequency area except the guard bands is divided into three segments for three sectors. A preamble is inserted at an interval of three subcarriers, with 0s filled at the other subcarriers. For example, a preamble for segment 0 is inserted at subcarriers 0, 3, 6, 9, . . . , 843, 846 and 849. A preamble for segment 1 is inserted at subcarriers 1, 4, 7, 10, . . . , 844, 847 and 850. A preamble for segment 2 is inserted at subcarriers 2, 5, 8, 11, . . . , 845, 848 and 851.
In the IEEE 802.16e system, a sequence used for the preamble is a binary code inserted in a frequency area, as proposed by Runcom. Among sequences that can be generated as binary codes, sequences that satisfy certain correlation properties and have low Peak-to-Average Power Ratios (PAPRs) in the time domain were found, by computer-aided search. Table 1 below lists some preamble sequences.
TABLE 1IndexIDcellSegmentSeries to modulate (in hexadecimal format)0000xA6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF1100x668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D64AB4A33C467001F3B22200x1C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE03E9B5AB5E1D3F98C6E3300x5F9A2E5CA7CC69A5227104FB1CC2262809F3B10D0542B9BDFDA4A73A7046096DF0E8D3D
Referring to Table 1, a preamble sequence is determined according to a segment number and a parameter IDcell. Each preamble sequence is converted to a binary signal and modulated by Binary Phase Shift Keying (BPSK), prior to mapping to subcarriers. Specifically, a hexadecimal sequence is converted to a binary sequence Wk and modulated by BPSK in the order from the Most Significant Bit (MSB) to the Least Significant Bit (LSB) (0=>+1, 1=>−1). For example, hexadecimal sequence 0 is converted to Wk[110000010010 . . . ] and then modulated to [−1−1+1+1+1+1+1−1+1+1−1+1 . . . ].
FIGS. 2 and 3 are examples of using sequences for signal transmission. Besides the preamble, sequences are used for transmission of many other channels and signals in the wireless communication system. For example, sequences have a variety of usages including synchronization channels, a midamble, a reference signal, control channels, scrambling codes, and multiplexing in the wireless communication system. The sequences preferably satisfy the following characteristics:                The sequences have good correlation properties to provide good detection performance;        The sequences have Low Cubic Metrics (CMs) or PAPRs to maximize the efficiency of a power amplifier;        Many sequences are available to facilitate transmission of a large amount of information or cell planning;        The sequences reduce a memory size requirement for storing the sequences; and        A receiver has a low complexity, when the receiver uses the sequences.        
Especially the complexity of the receiver is a very significant factor that directly affects its battery lifetime. Accordingly, there exists a need for a method for generating a sequence so as to further decrease a memory size requirement, while reducing complexity.